Save metrics grid dimensions and search with a Discover session

## Summary

Adds a framework for Discover data source profiles to persist their own state inside a saved Discover session, with a new tabTypeState field on the Discover session saved object. The Metrics profile is the first adopter: the selected dimensions, the metric search term, and the counter, gauge, and histogram percentile aggregation settings of a metrics tab now survive save, reopen, and duplicate instead of resetting to defaults, and changing them after saving surfaces the unsaved changes indicator. Dimensions and search are always available in the metrics toolbar. The aggregation settings flyout is gated by the discover.metricsExperienceEditGridSettingsEnabled feature flag, which defaults to false, so that part stays hidden for most users. The PR also stops force-syncing grid dimensions with the histogram breakdown field.

**Why this needs docs:** Labeled release_note:skip and framed as internal plumbing, but the user-visible outcome is that a saved Discover session with a metrics tab now restores its dimension selection and search term, which neither the Discover session page nor the metrics exploration page mentions.

## Resources

- PR [#285454](https://github.com/elastic/kibana/pull/285454) — [Discover] Support tab types and saved profile state

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Sep 7–Sep 11 |
| **Feature status** | preview |
| **Feature flag** | None — active by default |

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*Created with [Docs Quest Scanner](https://github.com/florent-leborgne/docs-quest-scanner) by @florent-leborgne*

## Suggested edits

[Explore metrics data with Discover in Kibana](https://www.elastic.co/docs/solutions/observability/infra-and-hosts/discover-metrics) > Metrics-specific Discover options
- **What the docs say:** With your data loaded, use the metrics charts to: Search for specific metrics ... Break down metrics by dimensions ...
- **What to add:** Add a short paragraph or list item stating that when you save the Discover session, the selected dimensions and the metric search term are saved with the metrics tab and restored when you reopen or duplicate the session. Lead the paragraph with an inline {applies_to} tag. Do not document the aggregation settings flyout, it is behind a feature flag that defaults to off. Applies from 9.6.0 and in serverless (technical preview).

[Save a Discover session for reuse](https://www.elastic.co/docs/explore-analyze/discover/save-open-search) > Save a Discover session
- **What the docs say:** By default, a Discover session stores the query text, filters, and current view of Discover, including the columns and sort order in the document table, and the data view.
- **What to add:** Optional one-line cross-reference: for metrics tabs, the session also stores the metrics view settings, with a link to the metrics exploration page. Keep the detail on the metrics page to match page scope to feature scope. Applies from 9.6.0 and in serverless (technical preview).